1. Field of the Invention
The present invention relates to a metal container lid, more particularly a sheet steel (or steel sheet) easy open can lid which enables part or substantially all of the can lid to be easily opened by hand, and is used for beverage cans, general food cans, and a wide range of other applications.
2. Description of the Related Art
Two types of easy open can lids enabling part or substantially all surface of the container lid to be easily opened by hand have been commercialized: the "tear off" type where a tab and an opening piece are torn off to separate the lid from the can body and the "stay on" type where the tab and the opening piece both remain affixed to the can body even after opening. In both types, almost all the easy open can lids are made of sheet aluminum due to reasons of manufacturing technology. Sheet steel is used at present for only some limited applications.
In a typical example of the prior art, coated aluminum or sheet steel is used as the material and is punched to the basic lid shape, then the lid body is placed on a flat lower mold half and a sharp blade having the required contour shape is pressed from the top surface so that the cutting edge bites into the lid body, thereby forming the shape of the opening piece surrounded by a V-sectional shaped tear-along groove (FIG. 6).
Steel materials themselves have a basic feature of a high strength. Forming a tear-along groove to an extent enabling easy opening by the hand requires strong pressure by the sharp blade corresponding to about one-half to two-thirds of the thickness of the sheet before processing. When the tear-along groove is too shallow in depth, the can openability is defective, while when too deep, an insufficient impact strength with respect to, for example, external shock are caused, and therefore, considerable accuracy has been considered necessary.
Therefore, considerable precision is required of the processing tools as well, but in the case of sheet steel requiring a strong pressure of the sharp blade, there was the defect that the tool life could not be maintained. Further, to ensure corrosion resistance with respect to the contents or to prevent rusting on the outside surface, repair coating of the portions of the metal surface exposed by the processing of the tear-along grooves was considered necessary.
As a means for extending the tool life, as shown in, for example, Japanese Unexamined Patent Publication Nos. 55-70434, and No. 57-175034, the method has been proposed of forming the tear-along grooves by composite cold forming. This known method was based upon the use of sheet steel and was an effective means of extending tool life, but there was the defect that since the sectional structure of the tear-along groove was complicated, with the normal spray coating method, the coating material would not reach all portions in the tear-along groove, and therefore, sufficient corrosion resistance could not be obtained, even with repair coating.
In the prior art using a sharp blade, aluminum was considered preferable as the material due to its properties, and sheet steel was used as a material only for extremely limited applications, as mentioned above. The reasons were mainly that (1) the resistance of sheet steel to strong pressure by a sharp blade is strong, and therefore, the life of the processing tools is extremely short, (2) the coating film on the surface of the sheet steel is broken by the processing and repair coating is required at the entire area of the tear-along groove or the tab calking portion, (3) scratches are sometimes generated in a coating film on the surface, which should be an inner surface of a can, during the processing, etc.
On the other hand, in recent years, there has been rising awareness of the global environment and to deal with this it is considered that an orientation toward recyclable products is necessary. In the area of metal cans, so-called "mono-metal cans" where the can body and the can lid are formed by the same material are being viewed with importance.
At the present time, the majority of metal cans use sheet steel as the material for the can bodies. There is a strong desire for some measure which will enable manufacture, with a good productivity, of sheet steel easy open can lids superior in can openability, not requiring repair coating of the inner or outer surfaces, and superior in corrosion resistance. Of course, sheet steel itself is superior in economy and by making both the can body and can lid out of sheet steel, the product can be expected to be more superior in economy and easier to recycle as a resource.